The technology described herein relates to an unducted thrust producing system, particularly a vane assembly paired with rotating elements. The technology is of particular benefit when applied to “open rotor” gas turbine engines.
Gas turbine engines employing an open rotor design architecture are known. A turbofan engine operates on the principle that a central gas turbine core drives a bypass fan, the fan being located at a radial location between a nacelle of the engine and the engine core. An open rotor engine instead operates on the principle of having the bypass fan located outside of the engine nacelle. This permits the use of larger fan blades able to act upon a larger volume of air than for a turbofan engine, and thereby improves propulsive efficiency over conventional engine designs.
Optimum performance has been found with an open rotor design having a fan provided by two contra-rotating rotor assemblies, each rotor assembly carrying an array of airfoil blades located outside the engine nacelle. As used herein, “contra-rotational relationship” means that the blades of the first and second rotor assemblies are arranged to rotate in opposing directions to each other. Typically the blades of the first and second rotor assemblies are arranged to rotate about a common axis in opposing directions, and are axially spaced apart along that axis. For example, the respective blades of the first rotor assembly and second rotor assembly may be co-axially mounted and spaced apart, with the blades of the first rotor assembly configured to rotate clockwise about the axis and the blades of the second rotor assembly configured to rotate counter-clockwise about the axis (or vice versa). In appearance, the fan blades of an open rotor engine resemble the propeller blades of a conventional turboprop engine.
The use of contra-rotating rotor assemblies provides technical challenges. One such challenge is transmitting power from the power turbine to drive the blades of the respective two rotor assemblies in opposing directions. A second challenge is minimizing the acoustic signature of the rotors. This is demanding because varied aircraft angles of attack cause the swirl angles into the rotor blades to vary circumferentially. The leading edges of blades with higher input swirl angles are loaded more heavily and tend to be more effective acoustic radiators of the noise of the upstream rotor. Another challenge, in part related to minimizing acoustic signature of the rotors, arises with installing the rotors on an aircraft. Rotor blades located near aircraft flow surfaces, including, for example, wings, fuselages, and pylons, can contribute to interaction penalties by disturbing the desired distribution of flow seen by the aircraft flow surface. This leads to suboptimal levels of resultant swirl into the wake of the aircraft and propulsion system and reduced propulsive efficiency.
It would be desirable to provide an open rotor propulsion system which more efficiently integrates with an aircraft.